Automated business machines for producing or reproducing hard copy documents, such as copiers, printers, facsimile machines, multi-function devices (MFD), and the like, often employ a scanning apparatus for scanning documents. Typically, a scanning apparatus will provide both manual document feed and automatic document feed capabilities. An automatic document feeder (ADF) is commonly used for automatically loading and unloading single sheets sequentially to a scanning station where the apparatus scans the fed document sheets for copying, faxing, displaying on a computer monitor, or the like. Following the operation, the ADF then off-loads a sheet and feeds the next sheet of the document to the scanning station.
Automatic document feeders often include a paper presence sensor that can detect whether or not a document is present in the input tray of the ADF. One common paper presence sensor employs a flag in conjunction with an optical sensor. When there is no paper loaded in the input tray of the ADF, the flag blocks the optical sensor; when paper is loaded in the input tray of the ADF, the flag is pushed out of view of the optical sensor. The controller of the scanning apparatus thus knows that paper is present when the optical sensor is not being blocked and paper is not present when the optical sensor is being blocked. When the start button of the scanning apparatus is pressed, the ADF will be activated if paper is present, but the ADF will not be activated if paper is not present.
The paper present flag drags on each sheet of the document as it is being fed into the ADF and thus produces friction. Because paper present flags are typically located off center with respect to the document sheet, the friction from the flag is off-centered and thus causes one side of the sheet to slow down with respect to the other side of the sheet. This causes skewing of the sheet (i.e., the sheet enters the ADF paper path at an angle). Skewing results in faulty scanning of the sheet.
One approach to preventing skewing is to include de-skew or registration rollers inside the ADF paper path. As a sheet is being feed into the ADF paper path, it contacts the de-skew rollers, which cause the sheet to buckle slightly so that the leading edge of the sheet lines up with the rollers before being advanced. However, this approach adds significantly to the cost of the apparatus as the additional rollers, plus a motor for the rollers, must be provided.
Another approach to preventing skewing is the provision of a skew correcting algorithm that adjusts for skewing. However, such algorithms tend to be very complex and many scanning apparatus that have ADFs do not employ enough computing power to digitally de-skew documents.